Overview

Comprehensive Description

Conopeum reticulum is an encrusting bryozoan, abundant in the lower intertidal zone and extending down to the shallow subtidal waters of Britain and Ireland. The species is also commonly found in estuarine and brackish environments. C. reticulum forms white patches, which resemble a thin sheet of gauze on stones and other hard-substrata.

Conopeum reticulum has been reported from the coasts of the southern North Sea, including those of the UK, Belgium and the Netherlands. It is thought to occur on most British coasts, but its distribution is imprecisely known.

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Description

Conopeum reticulum colonies form extensive gauze-like encrustations. Individuals (zooids) within the colony are approximately 0.4-0.6 x 0.2-0.3 mm in size, elongate, rectangular or polygonal in outline with a thickened, finely granular margin. The margin occasionally bears a few thin, pointed, delicate spines. The upper (frontal) surface of zooids is membranous with a semicircular light-brown operculum at one end. Triangular dwarf, non-feeding zooids are often present at the distal end of zooids in the gaps between the normal zooids.Normal autozooids occasionally become irregularly shaped or larger, especially at the edges of colonies where there are sometimes large, irregular gaps to fill.

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northern Gaspe waters, southern Gaspe waters (Baie des Chaleurs, Gaspe Bay to American, Orphan and Bradelle banks; eastern boundary: eastern Bradelle Valley), middle North Shore (from Sept- Iles to Cape Whittle, including the Mingan Islands); western slope of Newfoundland, including the southern part of the Strait of Belle Isle but excluding the upper 50m in the area southwest of Newfoundland

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The distribution of Conopeum reticulum is imprecisely known. It has been recorded from British coasts throughout the country, but is likely to be more common in Wales and south England. It is also known from Irish, Belgian and Danish shores. It is thought not to extend into the Mediterranean

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Physical Description

Morphology

Conopeum reticulum forms white encrusting gauze-like sheets, which often cover an extensive area. Zooids are irregularly oval in shape, and vary in size. The lateral walls of the zooids are heavily calcified, forming a raised rim around the zooid edge. The frontal surface is largely membranous. A narrow, granular section of calcification on the frontal surface (the gymnocyst and cryptocyst) is present, and is thicker than Conopeum seurati. The non-calcified section of the frontal surface (the frontal membrane) and the underlying space (opesia) are elliptical or oval in shape. Occasionally, a number of spines are present around the frontal area.

The operculum, a hinged flap which closes the orifice, is very characteristic in Conopeum species. It has a folded membranous edge and appears as a thick crescent-shaped structure when closed. It is lightly chitnizied and lacks the thin marginal sclerite typical of Membranipora species. In C. reticulum the operculum appears as a broad semicircle, which is light brown, when it is closed.

A pair of specialised triangular zooids (kenozooids), much smaller than other zooids and containing either no polypide or a minute polypide, is present on the distal edge (furthest from the colony origin) of feeding zooids. These kenozooids may vary from the triangular shape in parts of the colony and often become enlarged, apparently increasing in size to fill space along the colony edge which is caused by irregularities in the substratum.

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Look Alikes

Species of Conopeum are easily confused with those of the related genus Membranipora. Membranipora species may be distinguished by the presence of a twinned ancestrula (the founding zooid), compared to the single ancestrula of Conopeum species. However, the ancestrula region is frequently missing from colonies, making this character insufficient to distinguish species. Conopeum reticulum has thicker calcification than Membranipora species, and a more apparent gymnocyst and cryptocyst than C. seurati. C. reticulum may also be distinguished from other littoral and sublittoral species in the same family by the presence of paired triangular chambers (kenozooids) at the end of each zooid.

Conopeum and Membranipora species also differ in their ecology. M. membranacea is the only species of its genus to occur in British waters, where it forms extensive colonies, normally on Laminaria. Membranipora tuberculata, which colonises Sargassum, and Membranipora tenuis, a tropical species, are, on rare occasions, washed up on south-western shores of the UK. Both of the British species of Conopeum colonise hard substrata or estuarine plants, but would not be expected to occur on marine algae.

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Conopeum reticulum is often found in abundance in the lower intertidal zone, and extends down to shallow subtidal waters. The species can inhabit both marine and freshwater environments, unlike the other British species of Conopeum, C. seurati, which is restricted to brackish water. C. reticulum is able to colonise many different hard substrates and is not usually associated with algal substrates.

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Conopeum reticulum favours estuaries and bays on sheltered coasts. Found from the lower shore into the shallow sublittoral, on hard substrata, shells, stones, and submerged structures. Not usually associated with plants. In brackish areas, it is often abundant on the inner sides of oyster valves (Ostrea edulis). Rarely found off-shore.

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Life Cycle

The founding zooid (ancestrula) develops into a young colony, and later into an adult colony through asexual budding. Sexually produced embryos develop into larvae which are released into the plankton. Larvae settle after liberation and metamorphose into a single ancestrula.

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Reproduction

Each fertile zooid successively sheds a large number of eggs which are yellowish – white and measure 0.1 by 0.08 mm. The eggs and spermatozoa are produced from July to September. The larvae of C. reticulum are planktonic cyphonautes which feed and grow to 0.25 by 0.19 mm. The larvae are triangular in shape and laterally compressed and are common in coastal and estuarine plankton from throughout the summer, settling in the late summer and early autumn.